Demand for hot dip Zn plated steel sheets has been expanded in the fields of construction materials, home appliances, or automobiles because the manufacturing process thereof is simpler and the price thereof is lower than electro-galvanized steel sheets. In particular, according to a recent increase in the price of Zn, techniques related to a hot dip zinc-aluminum (hereinafter referred to as Zn—Al system) or a hot dip zinc-aluminum-magnesium (hereinafter referred to as Zn—Al—Mg system) alloy plated steel sheet having better corrosion resistance at a lower coating weight in comparison to a hot dip Zn plated steel sheet have been developed and demands for Zn—Al and Zn—Al—Mg systems have increased.
A typical Zn—Al system product may be a Zn-55% Al plated steel sheet. However, since aluminum content in a plating layer, there are limitations in that a sacrificial corrosion protection capability is decreased and corrosion preferentially occurs in a portion exposing underlying metal such as a cutting surface. Also, with respect to hot dip Zn-55% Al plating, the generation of dross in a plating bath may be severe, because the temperature of the plating bath may be high at about 600° C., plating workability may decrease and the lifespan of a facility may be shortened, due to the erosion of components of the facility in the plating bath, such as a sink roll.
With respect to the Zn—Al—Mg system, U.S. Pat. No. 3,505,043 was suggested, and thereafter, Japanese Patent Application Laid-Open Publication No. 8-60324, Japanese Patent Application Laid-Open Publication No. 10-226865, and Japanese Patent No. 3201469 were suggested. The foregoing Japanese patents disclosed that total contents of aluminum and magnesium in plating layers were in a range of 9 wt % to 14 wt %, and the plating layers showed appropriate quality characteristics for construction materials due to excellent corrosion resistance. However, the use of the plating layers for automobiles may be difficult because the surface qualities thereof may deteriorate due to high levels of alloying components, such as aluminum and magnesium, in the plating layers.
Also, a technique exists in Europe, in which the total contents of aluminum and magnesium in a plating layer may be controlled to a lower level in comparison to those in Japan. However, in this case, corrosion resistance may be somewhat decreased.
Meanwhile, in terms of manufacturing, when the contents of aluminum and magnesium are controlled to low levels, a solidification initiation temperature of a Zn—Al—Mg alloy plating layer may be in a range of 400° C. to 420° C., although the temperature may differ somewhat according to the contents of aluminum and magnesium. A final solidification termination temperature of a Zn—Al—Mg ternary eutectic structure is about 340° C. and surface qualities thereof may deteriorate due to the generation of ripple marks caused by a selective oxidation of magnesium in a liquid phase-solid phase temperature range. That is, aluminum and magnesium are concentrated in a molten metal pool unsolidified during a solidification process of the plating layer. The higher the concentration of magnesium is, the easier oxidation occurs and the more non-uniform the fluidity is.
Therefore, a technique for a hot dip Zn—Al—Mg system plated steel sheet, which may secure excellent corrosion resistance and uniform surface qualities while the contents of aluminum and magnesium in a plating layer are controlled to be as low as possible, is required.